1. Field of the Invention
The present invention relates to a lamella preparation method and a lamella preparation apparatus for preparing a lamella by etching processing using a focused ion beam.
2. Description of the Related Art
Conventionally, TEM observation is known as a method of observing a microscopic region in a sample for the purpose of analyzing a defect in a semiconductor device or the like. In the TEM observation, in order to obtain a transmission electron image, as preprocessing, it is necessary to process a sample into a lamella having a thickness through which an electron beam may transmit.
In recent years, as a method of preparing a lamella, a lamella preparation method using a focused ion beam is used. In this method, etching processing of peripheral portions of a sample is performed so as to leave a portion in the sample including a region of which observation is desired. Then, etching processing of the remaining portion is performed until the portion has a thickness through which an electron beam may transmit to prepare the lamella. This enables preparation of a lamella precisely including the region of which the observation is desired.
By the way, in the TEM observation, it is desired that the thickness be uniform throughout the lamella. When the thickness is nonuniform, the nonuniformity affects a TEM image and information of the defect and the effect of the nonuniformity in thickness cannot be separated, which causes the analysis to be difficult.
However, a focused ion beam has a certain beam density distribution due to the nature thereof, and thus, when a lamella is prepared by irradiating a focused ion beam from a surface side of a sample, the lamella is tapered, which means the thickness of the lamella is nonuniform.
As a method for solving such a problem, there is disclosed a method of preparing a lamella having a vertical thin wall by etching processing with the sample being slanted taking into consideration the beam density distribution of the focused ion beam (see Japanese Patent Application Laid-open No. Hei 4-76437). According to this method, a lamella having a uniform thickness irrespective of the beam density distribution of the focused ion beam may be prepared.
However, in recent years, as the density of semiconductor devices becomes higher and as the dimensions of a semiconductor device become smaller, the size of a defect which is a target of TEM observation also becomes smaller. In order to make the TEM observation of a microscopic defect with accuracy, it is necessary for the thickness of the lamella to be extremely small.
In conventional lamella preparation, in an apparatus including both a focused ion beam column and an electron beam column, SEM observation of both a front surface and a rear surface of a lamella is made to confirm the shape of the processed lamella. However, in the conventional method involving merely obtaining a SEM image and confirming nonuniformity in thickness by the contrast in the SEM image, when the thickness of the lamella is extremely small, the difference in contrast is small, and thus, it is difficult to confirm the nonuniformity in thickness.
Even if nonuniformity in thickness may be confirmed by the SEM observation, the slant angle of the sample stage necessary for processing for uniform thickness is unknown, and thus, an operator performs the processing while adjusting the slant angle. Such a method depends on the skill of the operator, and thus, it is difficult to secure a certain level of quality.